Copending U.S. patent application Ser. No. 07/544,415 entitled "Method Of Making Hollow Articles" of common assignee herewith describes a method of making a hollow article, for example, a hollow airfoil, by diffusion bonding first and second preformed sections (e.g., airfoil halves). The method involves initially forming a substantially fluid impermeable precursor diffusion bond (i.e., a gas seal bond) between opposing bonding surfaces of the first and second sections with only limited distortion of one of the sections and no distortion of the other section. In particular, in the initial precursor bonding step, the assembled sections are subjected to coordinated pressure-temperature-time conditions where a relatively low first external pressure (e.g. about 50-100 psi) is so applied to the first section with a cavity between the sections evacuated as to form a substantially fluid impermeable precursor or seal bond between the bonding surfaces During this step, an intermediate wall of the first section undergoes some inward distortion as a result of the negative pressure differential applied thereacross. Preferably, the extent of such inward distortion is controlled or limited to an amount which is removable by subsequent inflation of the intermediate wall. Preferably, distortion is limited by applying a relatively low external pressure (e.g., 50-100 psi) on the first section. If the airfoil design permits, the inward distortion of the intermediate wall is limited to prevent the intermediate wall from contacting the intermediate wall of the second section, thereby eliminating the need for a release agent on the intermediate walls. Distortion of the intermediate wall of the second section is prevented by subjecting both the cavity and the exterior of the second section to a relative vacuum while supporting the second section on a rigid die or shaping surface.
The first and second precursor or seal bonded sections are then subjected to coordinated pressure-temperature-time conditions where a relatively high external pressure of greater magnitude (e.g, 300-600 psi) than that applied to effect the precursor bond is applied to the second section and an internal fluid pressure of substantially the same magnitude (e.g., 300-600 psi) as the second external pressure is applied to the cavity while the intermediate wall of the first section is cooperatively positioned relative to a rigid shaping surface. The internal pressure in the cavity causes the intermediate wall of the first section to conform to the shaping surface to remove any distortion resulting from the previous seal bonding step. The external pressure on the second section, in turn, effects complete diffusion bonding between the bonding surfaces and prevents distortion of the intermediate wall of the second section.
Alternately, the diffusion bonding step and inflation steps of the method can be practiced separately. For example, the first and second precursor bonded sections are subjected to coordinated pressure-temperature-time conditions where the aforementioned relatively high second external pressure is so applied to the first section and an internal pressure of substantially the same magnitude is so applied to the cavity as to diffusion bond the bonding surfaces without further inward distortion of the intermediate wall of the first section During this high pressure diffusion bonding step, distortion of the intermediate wall of the second section is prevented by supporting it on a rigid die or shaping surface. After diffusion bonding, the sections are then subjected to a separate inflation step where a relatively high internal pressure (e.g, 300-600 psi) is applied to the cavity with the intermediate wall of the first section cooperatively positioned relative to another rigid die or shaping surface. The internal pressure is applied at a level (e.g., 300-600 psi) to cause the intermediate wall of the first section to conform to the rigid shaping surface to remove distortion. Distortion of the intermediate wall of the second section is prevented by applying external pressure thereon of substantially the same magnitude as the internal pressure applied in the cavity between the sections.
It is an object of the invention to provide a method of making a hollow article, such as a hollow airfoil, from preformed sections in accordance with the method described hereinabove wherein the external pressure is varied in such a way during the precursor seal bonding operation as to enhance contact between the opposing bonding surfaces
It is another object of the invention to provide a method of making a hollow article, such as a hollow airfoil, where, during the operation to form the precursor or seal bond, the external pressure is varied in such a way as to prevent deleterious distortion of the sections while enhancing contact between the opposing bonding surfaces.